Non-volatile memory using pyramidal nanocrystals as electron storage elements

ABSTRACT

A non-volatile memory device includes a floating gate with pyramidal-shaped silicon nanocrystals as electron storage elements. Electrons tunnel from the pyramidal-shaped silicon nanocrystals through a gate oxide layer to a control gate of the non-volatile memory device. The pyramidal shape of each silicon nanocrystal concentrates an electrical field at its peak to facilitate electron tunneling. This allows an erase process to occur at a lower tunneling voltage and shorter tunneling time than that of prior art devices.

TECHNICAL FIELD

The present disclosure relates generally to the manufacture ofsemiconductor devices, and more particularly, to the manufacture ofintegrated circuits having non-volatile memory devices.

BACKGROUND

Non-volatile memory (NVM) technology has faced challenges in attemptingto improve the writing/reading speed and injection efficiency of hotcarriers into the tunneling oxide of a memory cell. Non-volatile memory(NVM) devices that utilize a channel hot electron (CHE) injectionprocess are inefficient. This inefficiency results in a low writingspeed and a need for a large area to adequately perform a hot electroninjection process. Non-volatile memory (NVM) devices that utilize aFowler-Nordheim tunneling process are efficient. However, theFowler-Nordheim tunneling process has a low read performance.

This means that there is a fundamental limit on the speed and scaling ofconventional non-volatile memory (NVM) devices.

FIG. 1 illustrates a schematic diagram of a prior art memory cell 100 ofan electrically erasable programmable read only memory (EEPROM) device.Memory cell 100 includes one P-channel metal oxide semiconductor (PMOS)transistor 110 and one P-channel metal oxide semiconductor (PMOS)capacitor 120. The PMOS capacitor 120 is formed by connecting togetherthe source, drain and substrate of a PMOS transistor.

The PMOS transistor 110 may be referred to as PMOS program transistor110, while the PMOS capacitor 120 may be referred to as PMOS controlcapacitor 120. The gate of the PMOS program transistor 110 and the gateof the PMOS control capacitor 120 are connected together (i.e., shortedtogether) and are isolated from the other active elements. The shortedgates of the PMOS program transistor 110 and the PMOS control capacitor120 are collectively referred to as a “floating gate” 130. Charges (inamounts that represent either a zero (“0”) representation or a one (“1”)representation) may be written to the floating gate 130. In order toavoid well bias interference, the PMOS program transistor 110 and thePMOS control capacitor 120 are each located in a separate N well.

The prior art memory cell 100 is written to by injecting drain avalanchehot electrons into the floating gate 130. For PMOS operation (as shownin FIG. 1) a low voltage is applied to the control gate and drain ofPMOS control capacitor 120 and a high voltage is applied to thesource/well of PMOS program transistor 110. The channel of PMOS programtransistor 110 is turned on and hot electrons are generated at the highelectric field region at the drain junction (designated “V_(INJ)” inFIG. 1). With positive voltage on the control gate of PMOS controltransistor 120, some hot electrons with high energy will pass throughthe silicon-silicon dioxide (Si—SiO₂) potential barrier and be injectedinto the floating gate 130.

The prior art memory cell 100 is erased by applying a high voltage tothe control gate of the PMOS control transistor 120 and to the grounddrain and source of the PMOS program transistor 110. Electrons on thefloating gate 130 will pass through the gate oxide between the floatinggate 130 and the control gate of the PMOS control capacitor 120 byFowler-Nordheim (FN) tunneling process and into the substrate. Adescription of the physics of the Fowler-Nordheim (FN) tunneling processis set forth in U.S. Pat. No. 5,225,362, which is incorporated herein byreference.

FIG. 2 illustrates a prior art structure 200 that illustrates the use ofa plurality of silicon nanocrystals 210 on the surface of a tunnel oxide220 grown on a silicon substrate. The silicon nanocrystals 210 functionas the “floating gate.” The tunnel oxide 220 and the siliconnanocrystals 210 are covered with a gate oxide 230. A control gate 240is located above the gate oxide 230.

During the erase process, electrons will pass from the siliconnanocrystals 210 through the gate oxide 230 to the control gate 240 bythe Fowler-Nordheim (FN) tunneling process. The silicon nanocrystals 210facilitate the passage of the electrons through the gate oxide 230.

Prior art silicon nanocrystals 210 are typically either spherical orhemispherical. A typical hemispherical silicon nanocrystal geometry isshown in FIG. 2. The hemispherical silicon nanocrystal geometry exposesthe largest cross-section to the tunnel oxide 220 and provides the mostefficient charge injection from the substrate (not shown). The eraseprocess is less efficient likely because the electric field is uniformlydistributed on the top surface of the silicon nanocrystals 210.

Accordingly, there is a need in the art for an improved non-volatilememory (NVM) device (and method of manufacture) that increases the eraseefficiency while at the same time maintaining the advantages that areprovided by the hemispherical silicon nanocrystals.

SUMMARY

In accordance with one advantageous embodiment, a non-volatile memorydevice is provided that comprises a floating gate and a plurality ofpyramidal silicon nanocrystals that are associated with the floatinggate. Electrons are stored on the pyramidal silicon nanocrystals. Whenthe non-volatile memory device performs an erase process, the electronstunnel from the plurality of pyramidal silicon nanocrystals through agate oxide layer to a control gate of the non-volatile memory device.

A peak at the top of each pyramidal silicon nanocrystal concentrates anelectrical field at the peak to facilitate electron tunneling. Thisallows the erase process to be performed at a lower tunneling voltageand at a lower tunneling time than that of prior art devices.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure so that those skilled in the artmay better understand the detailed description that follows. Additionalfeatures and advantages of the disclosure will be described hereinafterthat form the subject of the claims. Those skilled in the art shouldappreciate that they may readily use the conception and the specificembodiment disclosed as a basis for modifying or designing otherstructures for carrying out the same purposes described herein. Thoseskilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure in its broadest form.

Before undertaking the Detailed Description below, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prioruses, as well as future uses, of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, wherein likenumbers designate like objects, and in which:

FIG. 1 illustrates a schematic diagram of a prior art memory cell of anelectrically erasable programmable read only memory (EEPROM) device;

FIG. 2 is a diagram illustrating a cross sectional side view of a priorart structure having a plurality of hemispherical silicon nanocrystalson the surface of a tunnel oxide of a floating gate;

FIG. 3 is a diagram illustrating a cross sectional side view of astructure used to manufacture the plurality of prior art hemisphericalsilicon nanocrystals shown in FIG. 2;

FIG. 4 is a diagram illustrating a cross sectional side view showing howa plurality of prior art hemispherical silicon nanocrystals are formedfrom the structure shown in FIG. 3;

FIG. 5 is a transmission electron microscope (TEM) photograph of a priorart hemispherical silicon nanocrystal;

FIG. 6 is a diagram illustrating a cross sectional side view of astructure that illustrates a plurality of pyramidal silicon nanocrystalson the surface of a tunnel oxide of a floating gate in accordance withthe present disclosure;

FIG. 7 is a diagram illustrating a cross sectional side view of astructure used to manufacture the plurality of pyramidal siliconnanocrystals shown in FIG. 6;

FIG. 8 is a diagram illustrating a cross sectional side view showing anintermediate step in the manufacture of a plurality of pyramidal siliconnanocrystals;

FIG. 9 is a diagram illustrating a cross sectional side view showing howa plurality of pyramidal silicon nanocrystals are formed from thestructure shown in FIG. 8;

FIG. 10 is a transmission electron microscope (TEM) photograph of aplurality of pyramidal silicon nanocrystals; and

FIG. 11 is a diagram illustrating a flowchart of an advantageousembodiment of a method of manufacturing pyramidal silicon nanocrystalsin accordance with the present disclosure.

DETAILED DESCRIPTION

FIGS. 3 through 11 and the various embodiments used to describe theprinciples of the present disclosure in this patent document are by wayof illustration only and should not be construed in any way to limit thescope of the invention. Those skilled in the art will understand thatthese principles may be implemented in any type of suitably arrangednon-volatile memory (NVM) device.

To simplify the drawings the reference numerals from previous drawingswill sometimes not be repeated for structures that have already beenidentified.

To better provide a thorough explanation of the technical advantages, ageneral description of the manufacture of prior art hemisphericalsilicon nanocrystals will first be given.

FIG. 3 is a diagram illustrating a cross sectional side view of anintermediate structure 300 used in the manufacture of the prior arthemispherical silicon nanocrystals 210 shown in FIG. 2. A siliconsubstrate 310 is provided with a tunnel oxide layer 320 formed (e.g.,grown) on or over the silicon substrate 310. A layer of amorphoussilicon 330 (also referred to as a-silicon 330) is formed (e.g.,deposited) over the tunnel oxide layer 320 and a rapid thermal anneal(RTA) process is performed to form a plurality of hemispherical siliconnanocrystals 410 by agglomeration, as shown in FIG. 4.

FIG. 5 is a transmission electron microscope (TEM) photograph of a priorart hemispherical silicon nanocrystal. As shown in FIG. 5, the length ofthe base of the hemispherical silicon nanocrystal is approximately ninenanometers (9 nm) and the height is approximately five nanometers (5nm). In general, a pyramidal silicon nanocrystal may have a base that isin a range of approximately eight nanometers (8 nm) to fifteennanometers (15 nm) and may have a height that is in a range ofapproximately eight nanometers (8 nm) to fifteen nanometers (15 nm).

FIG. 6 is a diagram illustrating a cross sectional side view of astructure 600 that illustrates a plurality of pyramidal siliconnanocrystals 610 disposed on the surface of a tunnel oxide 620 of afloating gate. The tunnel oxide 620 and the pyramidal siliconnanocrystals 610 are covered with a gate oxide 630. A control gate 640is positioned above the gate oxide 630. Because the silicon nanocrystals610 are three dimensional structures they are described as beingpyramidal. Transmission electron microscope (TEM) pictures show that thesilicon nanocrystals 610 have a non-rounded point (or vertex) at thetop. The silicon nanocrystals 610 can be generally described aspolyhedral nanocrystals with an upwardly pointing vertex.

During the erase process, electrons present on the silicon nanocrystalswill pass from the tips of pyramidal silicon nanocrystals 610 throughthe gate oxide 630 to the control gate 640 by the Fowler-Nordheim (FN)tunneling process. The configuration and structure of the pyramidalsilicon nanocrystals 610 facilitate the passage of the electrons throughthe gate oxide 630.

The shape of the pyramidal silicon nanocrystals 610 provides ananocrystal peak thereon which is located adjacent to the control gate640. It will be appreciated that electrons are able to tunnel throughthe gate oxide 630 from a peak of a pyramidal silicon nanocrystal 610 ata lower applied voltage than required using the prior art hemisphericalsilicon nanocrystal 210. This is because of the concentration of fieldlines at the peak of the pyramidal silicon nanocrystal 610 and thecontrol gate 640. In contrast, the electric field lines in the case of aprior art hemispherical silicon nanocrystal 210 are more uniformlydistributed over the top surface of the hemispherical siliconnanocrystal 210. The pyramidal geometry facilitates electron tunnelingand allows a reduction in erase voltage as compared to prior art erasevoltages. This also allows the erase time to be reduced compared toprior art erase times.

In addition, it will be appreciated that the thickness of the gate oxide630 may be chosen or selected to prevent program disturb and readdisturb effects. The use of pyramidal-shaped silicon nanocrystalsimproves the efficiency of the erase process (in terms of lower erasevoltages and lower erase times) while essentially maintaining theadvantages provided by the prior art hemispherical silicon nanocrystals210.

FIG. 7 is a diagram illustrating a cross sectional side view of anintermediate structure 700 used in the manufacture of thepyramidal-shaped silicon nanocrystals 610 shown in FIG. 6. A siliconsubstrate 710 is provided with a tunnel oxide layer 720 formed (e.g.,grown) on or over the silicon substrate 710. A layer of a-silicon 730 isformed (e.g., deposited) over the tunnel oxide layer 720 to form theresulting structure 700, as shown in FIG. 7.

An oxide deposition process is performed to form a deposited oxide layer820 over a plurality of brick-shaped (e.g., rectangular-shaped orrectangular cuboid) silicon nanocrystals 810, as shown in FIG. 8. Theoxide deposition process is followed by a rapid thermal anneal (RTA)process and a furnace anneal. The a-silicon 730 and the tunnel oxide 720can be sputtered or CVD deposited. The rapid thermal anneal (RTA) can bedone at a temperature between eight hundred degrees Celsius (800° C.)and nine hundred degrees Celsius (900° C.) for up to sixty seconds (60sec). The furnace anneal can be fifteen minutes (15 min) of annealing ata temperature of one thousand fifty degrees Celsius (1050° C.) Duringthe anneal, the a-silicon 730 recrystallizes to form the nanocrystals810.

An anneal process, such as a steam anneal process, is performed to formthe pyramidal-shaped silicon nanocrystals 910 from the brick-shaped(e.g., rectangular-shaped or rectangular cuboid) silicon nanocrystals810. In one embodiment, the anneal process includes wet oxidation at atemperature of approximately nine hundred and fifty degrees Celsius(950° C.) in a fifteen to twenty percent (15%-20%) diluted water vaporfor about fifteen minutes.

The resulting structure 900 and the resulting pyramidal-shaped siliconnanocrystals 910 after stripping off the top oxide are shown in FIG. 9.A gate oxide layer (not shown in FIG. 9) is then formed over the tunneloxide 720 and the pyramidal silicon nanocrystals 910. A control gatelayer (not shown in FIG. 9) is then formed over the gate oxide layer.

The sequence of processing as described herein is compatible withconventional processing for nanocrystal non-volatile memory (NVM)formation. The size of the pyramidal silicon nanocrystals 910 depends onthe layer thicknesses and the anneal conditions. In one advantageousembodiment of the invention, the size and thickness of the pyramidalsilicon nanocrystals 910 is approximately ten nanometers (10 nm).

FIG. 10 is a transmission electron microscope (TEM) photograph 1000 of aplurality of pyramidal silicon nanocrystals 610, 910. The pyramidalsilicon nanocrystals 610, 910 shown in FIG. 10 are of a suitable sizefor non-volatile memory (NVM) applications. In one embodiment, thepyramidal silicon nanocrystals 610 have a base having a length of abouttwelve nanometers (12 nm) and a height of about fourteen nanometers (14nm). Other suitable dimensions may be utilized.

FIG. 11 is a diagram illustrating a flowchart 1100 of an advantageousembodiment of a method of manufacturing the pyramidal siliconnanocrystals 610, 910. The flowchart 1100 summarizes the manufacturingsteps that have been previously described.

A silicon substrate 710 is provided (step 1110), a tunnel oxide layer720 is formed thereover (step 1120), and a layer of a-silicon 730 isformed over the tunnel oxide layer 720 (step 1130). A series ofdeposition and anneal steps is performed to form the pyramidal-shapedsilicon nanocrystals. In one embodiment, this process includes an oxidedeposition process and a rapid thermal anneal (RTA) process which formsa plurality of brick-shaped (e.g., rectangular-shaped or rectangularcuboid) silicon nanocrystals 810 (step 1140) and an anneal process(e.g., steam anneal) that subsequently forms the pyramidal-shapedsilicon nanocrystals 910 from the brick-shaped (e.g., rectangular-shapedor rectangular cuboid) silicon nanocrystals (step 1150).

A gate oxide layer is formed over the tunnel oxide layer 720 and thepyramidal silicon nanocrystals 910 (step 1160) and a control gate layeris formed over the gate oxide layer (step 1170).

It will be understood that well known processes have not been describedin detail and have been omitted for brevity. Although specific steps,structures and materials may have been described, the present disclosuremay not be limited to these specifics, and others may be substituted asis well understood by those skilled in the art.

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

What is claimed is:
 1. A method of operating a non-volatile memory device wherein the method comprises the steps of: providing at least one pyramidal silicon nanocrystal within a floating gate of the non-volatile memory device, the at least one pyramidal silicon nanocrystal formed during formation of the floating gate; performing an erase operation in which electrons tunnel through a gate oxide layer of the non-volatile memory device from the at least one pyramidal silicon nanocrystal; wherein the pyramidal silicon nanocrystal comprises an upward pointing peak that concentrates an electrical field at the peak of the pyramidal silicon nanocrystal; and wherein the peak of the pyramidal silicon nanocrystal allows electrons to tunnel through the gate oxide layer at a lower tunneling voltage than that required for electrons tunneling from a hemispherical silicon nanocrystal.
 2. The method that is set forth in claim 1 wherein the peak of the pyramidal silicon nanocrystal allows electrons to tunnel through the gate oxide layer with a shorter tunneling time than that required for electrons tunneling from a hemispherical silicon nanocrystal.
 3. The method as set forth in claim 1 wherein each of the plurality of pyramidal silicon nanocrystals has a base that is approximately twelve nanometers (12 nm) and a height that is approximately fourteen nanometers (14 nm).
 4. A method of operating a non-volatile memory device, the method comprises: providing non-volatile memory device having a floating gate; providing within the floating gate a plurality of pyramidal silicon nanocrystals operable for storing electrons, the plurality of pyramidal silicon nanocrystals formed during formation of the floating gate; performing an erase operation in which the electrons tunnel through a gate oxide layer of the non-volatile memory device from the plurality of pyramidal silicon nanocrystals; wherein each of the plurality of pyramidal silicon nanocrystals comprises an upward pointing peak that concentrates an electrical field at the peak of the pyramidal silicon nanocrystal; and wherein the peaks of the pyramidal silicon nanocrystals allow electrons to tunnel through the gate oxide layer at a lower tunneling voltage than that required for electrons tunneling from a hemispherical silicon nanocrystal.
 5. The method that is set forth in claim 4 wherein the peaks of the pyramidal silicon nanocrystals allow electrons to tunnel through the gate oxide layer with a shorter tunneling time than that required for electrons tunneling from a hemispherical silicon nanocrystal.
 6. A method of forming a floating gate structure of a non-volatile memory device, the method comprising: forming a tunnel oxide layer on a silicon substrate; depositing amorphous silicon over the tunnel oxide layer; forming a plurality of silicon nanocrystals from the amorphous silicon, shaping each of the plurality of silicon nanocrystals into a pyramidal shape by performing a wet oxidation anneal process; forming a gate oxide layer over the pyramidal shaped silicon nanocrystals to form a floating gate; and forming a control gate layer over the floating gate.
 7. The method as set forth in claim 6 wherein forming the plurality of silicon nanocrystals comprises: depositing oxide over the amorphous silicon; and performing a thermal anneal process to form the silicon nanocrystals.
 8. The method as set forth in claim 7 wherein performing a thermal anneal process comprises: performing a rapid thermal anneal process; and performing a furnace anneal.
 9. The method as set forth in claim 8 wherein the rapid thermal anneal process is to a temperature of between about 800 degrees Celsius and about 900 degrees Celsius. 